1. Field of the Invention
The present invention relates to a microwave-assisted magnetic recording method and a magnetic recording and reproducing apparatus which use a high frequency field in combination with a writemagnetic field from a write head.
2. Description of the Related Art
Miniaturization of bit size of recording media advances along with an increase in recording density of HDDs (Hard disk drives). However, with the bit size miniaturization advancing, loss of a recording state due to thermal fluctuation is of concern. While use of recording media with high magnetic coercivity (in other words, with high magnetic anisotropy) is needed to solve such a problem and maintain recording bits in stable condition in the high-density recording in the future, performing recording on a recording medium with high magnetic coercivity requires a strong writemagnetic field. However, in practice, due to downsizing of a write head and limitations on usable magnetic materials, there is also an upper limit on the strength of a writemagnetic field. For these reasons, the magnetic coercivity of recording media is restricted by the strength of a writemagnetic field that can be generated by a write head. Thus, in order to meet conflicting demands for media achieving high thermal stability while having magnetic coercivity which allows recording easily, recording methods have been devised to effectively reduce magnetic coercivity of a recording medium only during recording by using various aids. A typical method is thermally-assisted magnetic recording that is recording by using a magnetic head and a laser and the like in combination.
Meanwhile, there is also a method of performing recording by using a high frequency magnetic field in combination with a writemagnetic field from a write head and thereby locally reducing the magnetic coercivity of a recording medium. For example, Patent Document 1 discloses a technology for recording information by locally reducing the magnetic coercivity of a magnetic recording medium with the medium heated by Joule heating or magnetic resonance heating with the high frequency field. Such a recording method using the magnetic resonance of a high frequency field and the magnetic field from a magnetic head is hereinafter referred to as microwave-assisted magnetic recording. Using the magnetic resonance, the microwave-assisted magnetic recording requires application of a strong high frequency field, which is proportional to a magnetic anisotropy field of a medium, to produce the effect of reducing a switching field.
In recent years, the principle of generation of high frequency fields using spin torques like a spin-torque oscillator have been proposed, and the microwave-assisted magnetic recording has come to have a realistic possibility. For example, Non-Patent Document 1 discloses calculation results of a spin-torque oscillator which oscillates without any bias magnetic field from the external. In addition, Non-Patent Document 2 discloses a technology for recording information on a magnetic recording medium with high magnetic anisotropy by a perpendicular magnetic head and a field generation layer (FGL). The FGL is provided in the vicinity of the magnetic recording medium adjacent to a main pole of the perpendicular magnetic head and is configured to generate a magnetic field with fast magnetization rotation by spin torque. The FGL generates microwaves (high frequency field) during recording. Furthermore, Non-Patent Document 3 presents a spin-torque oscillator which controls a rotating direction of an FGL by utilizing a magnetic field of a main pole adjacent to the FGL. It is believed that these techniques can effectively leads to practical use of microwave assisted magnetization switching of media.    [Patent Document 1] JP 6-243527 A    [Non-Patent Document 1] X. Zhu and J. G. Zhu, “Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current,” IEEE Transactions on Magnetics, P2670 VOL. 42, No. 10 (2006)    [Non-Patent Document 2] J. G. Zhu, X. Zhu and Y. Tang, “Microwave Assisted Magnetic Recording,” IEEE Transactions on Magnetics, p. 125, vol. 44, No. 1 (2008)    [Non-Patent Document 3] Y. Wang, Y Tang and J. G. Zhu, “Media damping constant and performance characteristics in microwave assisted magnetic recording with circular ac field,” Journal of Applied Physics, 105, 07B902 (2009)